How does one calculate the maximum slow phase velocity of a nystagmus beat?
During ENG, the examiner needs to know the nystagmus intensity to correctly interpret the findings. Most computerized systems automate this task but there are times when the examiner has to manually measure the intensity. Those measurements are usually required only when the nystagmus has clear fast and slow phases. The following figure shows an example of horizontal nystagmus with rightward slow phases and leftward fast phases. This is referred to as leftbeating since nystagmus is identified by its fast phases. The nystagmus intensity is defined as the slow-phase velocity (SPV). To measure SPV for one nystagmus beat, the examiner has to determine the distance that the eye travels during the slow phase and divide that by the amount of time (SPV = ΔΘ/Δt). An easier method is to draw a line that best fits the slow phase of the nystagmus and measure the slope of this line. That way, the eye movement can be measured for a preset time, usually one second (SPV = ΔΘ’/1). To complete the proce
